1. Field of the Invention
The present invention relates to a surface acoustic wave device (hereinafter referred to as a “SAW device”), a duplexer comprising the SAW device, and a communications equipment carrying the duplexer
2. Description of Related Art
In recent years, as devices used for equipment utilizing radio waves, e.g., filters, delay lines, oscillators, etc. in communications equipment, SAW devices having features of being small in size and light in weight and having a steep cutoff performance as the filters have been frequently employed.
The SAW devices have been frequently used for filters in RF stages and IF stages of cellular phones, for example. The SAW devices have been also frequently used for duplexers that are circuits just below antennas.
The SAW devices are required to have low losses, have high attenuation characteristics outside pass bands, and have wide pass bands.
FIGS. 8 and 9 are graphs showing pass characteristics in a case where a SAW device is used as a filter in a duplexer that respectively pass through a transmission band and a receiving band. In this example, the frequency in the receiving band is higher than the frequency in the transmission band.
The attenuation in the pass band of the SAW device is referred to as an “insertion loss” or simply a “loss”. Further, the attenuation characteristics of a signal outside the pass band is referred to as “attenuation characteristics”. The amount of leakage of a signal in a transmission circuit into a receiving circuit is referred to as “isolation”.
A duplexer is required to have a low insertion loss in the transmission band and is also required to have high isolation between the transmission side and the receiving side.
As one type of SAW device, a ladder-type SAW filter in which a plurality of SAW resonators are disposed on a piezoelectric substrate and connected in a ladder shape as a whole in series and in parallel with a signal line has been known.
The ladder-type SAW filter has started to be widely employed as SAW filters for transmission and receiving iii a duplexer such as a cellular phone because it is small in size, has a low loss, and has sleep attenuation characteristics.
In a case where the SAW device is employed for the duplexer, it has so far examined that the ratio (L/P) of the width L of electrode fingers in an Inter Digital Transducer (IDT) electrode in the SAW resonator to the pitch (period) P between the electrode fingers is set to more than 0.5 for the purpose of improving the precision of the formation process of the SAW device (see JP 2001-308671 A).
On the other hand, in order to obtain good steep attenuation characteristics, it has been assumed that the ratio (L/P) of the width L of electrode fingers in an IDT electrode in a SAW resonator to the pitch P between the electrode fingers may be set to less than 0.5 (see JP 2003-198317 A).
In technology disclosed in JP 2001-308671 A, however, the above-mentioned ratio L/P is set to more than 0.5 so that the width L of the electrode fingers is widened if the pitch P between the electrode fingers is fixed. Therefore, spacing (P-L) between the electrode fingers is narrowed. Thus, the static capacitance of the SAW resonator is increased. As a result, the insertion loss of the SAW device is increased.
Since the ladder-type transmission SAW device in the duplexer is required to have a lower loss in a transmission frequency band, the duplexer has a high loss in the technology disclosed in JP 2001-308671 A.
In technology disclosed in JP 2003-198317 A, the above-mentioned ratio L/P is set to less than 0.5 so that the width L of the electrode fingers is narrowed. Therefore, spacing (P-L) between the electrode fingers is widened. Thus, the static capacitance of the SAW resonator is reduced. As a result, the attenuation outside the pass band, i.e., isolation of the SAW device is reduced.
In the ladder-type SAW device for receiving in the duplexer, an improvement in attenuation characteristics in a transmission frequency band is indispensable in order to realize high isolation characteristics. In the technology disclosed in JP 2003-198317 A, the duplexer becomes inferior in isolation characteristics.
It is generally known that the number of pairs of electrode fingers in the SAW resonator and the cross width of the electrode fingers may be increased in order to make the insertion loss of the SAW device low and make the attenuation characteristics thereof steep.
As the number of pairs of the electrode fingers and the cross width of the electrode fingers increase, however, the size of the SAW resonator increases. Therefore, a request in a market to achieve miniaturization cannot be satisfied.
An object of the present invention is to provide a SAW device that is low in loss and high in attenuation while being miniaturized, a duplexer using the SAW device, and communications equipment using the duplexer.